(1) Field of the Invention
This invention relates to a semi-conductor light-emitting element, particularly usable for a white color light-emitting diode.
(2) Related Art Statement
Recently, various light-emitting diodes (LEDs) are widely available. LEDs are expected for illuminating use as well as displaying use because of their low electric power consumption, long life time, CO2 gas reduction originated from the reduction of the high energy consumption such as the low electric power consumption, and thus, much demand for the LEDs are expected.
As of now, the LEDs are made of various semi-conducting material such as GaAs-based semi-conducting material, AlGaAs-based semi-conducting material, GaP-based semi-conducting material, GaAsP-based semi-conducting material and InGaAlP-based semi-conducting material, and thus, can emit various color lights from red to yellow-green. Therefore, the LEDs are employed particularly for various displaying use. Recently, blue and green LEDs have been realized by using GaN-based semi-conducting material. As a result, selecting a given LED, a given color light from red to blue, that is, within visible light range, can be obtained from the LED, and full-color displaying is also realized.
Moreover, white color light-emitting diodes (white color LEDs) are being realized by using RGB LED chips or using two color lights-emitting diodes composed of blue LEDs with yellow fluorescent substance thereon. As a result, LED illumination is being realized at present.
However, the white color LED using the RGB LED chips requires higher cost because the plural LED chips are employed, so that in view of the cost, it is difficult to employ the white color LED for illumination use. On the other hand, full color can not be recognized by the white color LED using the only color lights-emitting diode such as a combination of a blue LED and a yellow-fluorescent material because it employs only two primary colors, not three primary colors. Moreover, in the white color LED, the brightness of only about 25 lm/w can be realized, which is very small as compared with the brightness of 90 lm/W of a fluorescent tube.
Therefore, a white color LED employing three primary colors is strongly desired all over the world because of the low energy consumption taking environmental problem into consideration. In reality, such a white color LED is intensely developed by Japanese national projects and foreign major electric-manufacturing enterprises.
Such an attempt is made as to fabricate a white color LED using three or over primary colors as illuminating a three primary colors-fluorescent substance by an ultraviolet LED. This attempt is fundamentally based on the same principle as a fluorescent tube, and employs the ultraviolet LED as the ultraviolet beam from the mercury discharge in the fluorescent tube. In this case, the cost of the white color LED is increased because the three primary colors-fluorescent substance is additionally employed for the ultraviolet LED.
Using a GaN-based semi-conducting material, a blue LED can be realized, and then, using the GaN-based semi-conducting material, the ultraviolet LED can be realized. However, the luminous efficiency of the resulting ultraviolet LED is largely reduced, as compared with the blue LED.
As a result, the high luminous efficiency in such a white color LED as employing three or over primary colors through the illumination of a fluorescent substance by an ultraviolet LED is not technically prospected.
It is an object of the present invention to provide a new semi-conductor light-emitting element preferably usable for a LED to emit an any color light regardless of the dislocation density, particularly a white color LED.
For achieving the above object, this invention relates to a semi-conductor light-emitting element including a substrate, an underlayer, formed on the substrate, made of a first semi-conducting nitride material including at least Al element, a first conductive layer, formed on the underlayer, made of a second semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a first cladding layer, formed on the first conductive layer, made of a third semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, a light-emitting layer, formed on the first cladding layer, made of a fourth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In and at least one element selected from rare earth metal elements and transition metal elements as an additive element, a second cladding layer, formed on the light-emitting layer, made of a fifth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In, and a second conductive layer, formed on the second cladding layer, made of a sixth semi-conducting nitride material including at least one element selected from the group consisting of Al, Ga and In.
In this case, if the function of a conductive layer is applied to the first cladding layer, the first conductive layer may be omitted. Similarly, if the function of a conductive layer is applied to the second cladding layer, the second conductive layer may be omitted.
Conventionally, an underlayer of a semi-conductor light-emitting element is mainly used as a buffer layer, so the crystallinity of the underlayer is not important. Moreover, in the light of the buffer layer, it is desired that the crystallinity of the underlayer is degraded to some degree.
On the other hand, in a semi-conductor light-emitting element, the conductive layers, the cladding layers and a light-emitting layer are epitaxially grown on the underlayer by an MOCVD method. Therefore, the crystallinities of the layers are reflected and affected by the crystallinity of the underlayer, and if the crystallinity of the underlayer is low, the crystallinities of the layers are also lowered. As a result, the luminous efficiency of the semi-conductor light-emitting element can not be developed sufficiently.
On the contrary, the inventors found out that, in a semi-conductor light-emitting element made of Al-including semi-conducting nitride materials, even if the crystallinity of the underlayer, which is made of an Al-including semi-conducting nitride material, is increased to some degree, the underlayer can function as a buffer layer sufficiently.
Accordingly, as mentioned above, by enhancing the crystallinity of the underlayer up to 90 seconds or below in full width at half maximum (FWHM) of X-ray rocking curve, according to the present invention, the crystallinities of the light-emitting layer or the like of the semi-conductor light-emitting element, which are formed on the underlayer, can be also developed. In addition, the high crystallinity underlayer can function as a buffer layer. As a result, the luminous efficiency of the semi-conductor light-emitting element can be developed sufficiently. Herein, the FWHM is measured at (002) reflection.
In this case, however, it is difficult to obtain an any color light, particularly a white color light from the resulting semi-conductor light-emitting element. Therefore, the inventors conceived that at least one element selected from rare earth metal elements and transition metal elements is incorporated as an additive element into the light-emitting layer.
When a rare earth metal element or a transition metal element is excited by a given external energy, it would emit a light having its inherent wavelength. Therefore, if such a rare earth metal element and/or a transition metal element is incorporated in the light-emitting layer as an additive element, it may be excited by the luminescence of the light-emitting layer, and thus, generate and emit a given light with its inherent wavelength, that is, a given color light.
Accordingly, if an additive element is appropriately selected from rare earth metal elements and transition metal elements, and then, incorporated, the light-emitting layer can be emit a given color light originated from the excited inherent color light of the additive element.
Moreover, if plural elements, for example, Tm element to generate a blue color light wavelength-region light, Er element to generate a green color wavelength-region light, Cr, Eu or Pr element to generate a red color wavelength-region light are incorporated into the light-emitting layer, these inherent color lights are superimposed, to generate and emit a white color light from the light-emitting layer.
In such a semi-conductor light-emitting element as made of Al-including semi-conducting nitride material according to the present invention, the bandgap of a semi-conductor layer constituting the element is increased as the Al content of the semi-conductor layer is increased. Then, the luminous efficiency of the light-emitting layer is increased as the bandgap of the semi-conducting nitride material of the light-emitting layer is increased.